1. Field of the Invention
The present invention relates to a fixing rotating body such as a heating roller or an endless belt, which is coated with a fluororesin and can be used in a fixing apparatus for fixing unfixed toner images in copier, facsimile, laser beam printer and the like.
2. Description of the Related Art
In copier, laser beam printer and the like, unfixed toner images formed on a recording medium such as paper have hitherto been fixed by heat and pressure at a nip portion formed by a heating roller and a pressure roller in a fixing apparatus. Accordingly, a surface of the heating roller which directly contacts with a toner is coated with a fluororesin such as a perfluoroalkoxy resin (hereinafter briefly referred to as PFA) or polytetrafluoroethylene (hereinafter briefly referred to as PTFE), in order to prevent an offset caused by the toner from occurring.
However, the fluororesin is difficult to adhere to a metal used as a core of the heating roller, so that it is necessary to apply a primer. Accordingly, in the preparation of the heating roller, a primer layer is formed on a surface of the metal core as a layer which has a thickness of 5 to 10 μm, and further, a fluororesin layer is formed thereon as a coating layer which has a thickness of 20 to 30 μm. Thereafter, grinding is performed in order to control the surface roughness of the fluororesin layer. Accordingly, the total layer thickness of the heating roller becomes 20 to 30 μm.
Recently, from the energy-saving problem and the like, a halogen heater is energized only at the time of fixing to elevate the heating roller to a required fixing temperature, so that the heating roller excellent in thermal conductivity has become to be demanded. However, in the conventional heating roller, the core is coated with primer layer and the fluororesin layer poor in thermal conductivity to a total layer thickness of 20 to 30 μm. Accordingly, the heat of the halogen heater is not efficiently conducted to a surface of the heating roller, which causes the problem that the time taken until the heating roller reaches the surface temperature necessary for fixing the toner to the recording medium (hereinafter briefly referred to as the rising time) is slow. In order to solve this problem that the rising time of the heating roller is slow, a total layer thickness of the heating roller is 20 μm or less, preferably 17 μm or less, is considered to be necessary.
Consequently, in order to cope therewith, there has been used the method of grinding the fluororesin layer of the heating roller to a total layer thickness of 20 μm or less. However, the rising time is demanded to be further shortened, and it has become difficult to solve this problem only by decreasing the total layer thickness to 20 μm or less. On the other hand, there is a method which copes with this problem by decreasing the thickness of the metal core of the heating roller. However, when the heating roller using this thinned metal core is tried to be ground, the new problem that the metal core is deformed arises. It has therefore become very difficult to decrease the total layer thickness to 20 μm or less by grinding.
On the other hand, as a method for coating the core with the fluororesin whose total layer thickness is 20 μm or less without grinding, it has been proposed to solve this problem by dip coating using a coating containing an adhesive component (hereinafter briefly referred to as a “binder”) and necessitating no primer (hereinafter briefly referred to as a “one-coat coating”) (JP-A-11-194640). According to this method, there is obtained the heating roller having a fluororesin-mixed coating layer (hereinafter briefly referred to as a “coating layer”) of 20 μm or less in the total layer thickness on the metal core decreased in thickness.
As the fluororesin used in such a one-coat coating, PTFE is used.
However, PTFE is good in abrasion resistance, but high in melt viscosity to cause insufficient fusion among resin particles, so that a number of pores of 1 to 3 μm in size occur in the coating layer. There is therefore the problem that the melted toner or paper particles enter these pores to generate roller contamination and an offset.
On the other hand, as the fluororesin used in the one-coat coating, PFA is also known. PFA is low in melt viscosity to cause sufficient fusion among the resin particles, so that the coating layer has a smooth surface and good releasing ability. However, PFA is poor in abrasion resistance, so that there is the problem that abrasion scratches occur by the contact with paper sheet edges or separating claws, resulting in an inferior image having the scratches transferred thereon.
Further, in the case of the one-coat coating, when improvement of adhesive strength between the coating layer and the metal core is intended by increasing the binder in the coating, releasing ability of the coating layer deteriorates. On the other hand, when improvement of releasing ability of the coating layer is intended by decreasing the binder in the coating, adhesive strength between the coating layer and the metal core deteriorates. There is therefore also the problem that it is difficult to keep a balance of ratio between the fluororesin and the binder.
Like this, the heating roller using the conventional one-coat coating can be decreased to 20 μm in thickness of the coating layer. However, when it is used in a machine, abrasion resistance or releasing ability thereof is inferior, so that it has been desired that both abrasion resistance and releasing ability are satisfied.
Further, in full-color copier and printer, the conventional roller fixing takes too much rising time until the roller reaches the fixing temperature. Accordingly, a belt type fixing apparatus using an endless fixing belt has recently been proposed as a fixing apparatus which can shorten this rising time. However, there are commands for further shortening of the rising time, increased paper passing speed and increased durability of the fixing belt. Further, a fluororesin layer on a surface of the fixing belt also has the same problems as with the heating roller, and resolution thereof has been desired.